Detectors responsive to electromagnetic radiation generally include an array of pixels, each pixel being operable to generate or pass a current in response to electromagnetic energy incident upon the pixel. Typically, the generated or passed current is proportional to the incident energy. The pixels may be prone to some fluctuation in their response behavior over the life of the detector. An unknown change in performance of the detector could lead to erroneous data regarding measurements of an electromagnetic energy source. Accordingly, regular calibration is desirable to ensure uniformity in measurements. By determining the relative operating performance, calibration is used to correct for pixel to pixel variations (non-uniformity correction) and to correct for changes in the pixel response over time.
Generally, the methods of calibration include placing one or more sources at or near the detector to illuminate the detector. Although calibration in a laboratory environment is typically performed before deployment, regular re-calibration after deployment may be desired to accommodate for changes in pixel response over time.
As detector assemblies of this type are frequently used in space-based imaging devices, a complex calibration system is quite undesirable. Size and weight are important factors in the cost of assembly and launch of these space-based devices. Generally, an increase in size and weight of one component affects the size and weight of the remaining components. Further, the calibration system is typically designed to withstand the forces encountered during launch and deployment and then repeated operation requests without being easily serviceable. There are also a number of existing elements that are desired for detector functionality, such as spectral filter wheels and guidance systems. Providing calibration without compromising existing systems has proven challenging as well.
FIGS. 1 and 2 illustrate a conventional system 100 used for calibrating detector 102. FIG. 1 illustrates the calibration system 100 in a non-calibration mode of operation, while FIG. 2 illustrates the calibration system 100 in a calibration mode of operation. System 100 includes a source assembly 108 having one or more light sources 112 (e.g., light sources 112A-C), a mirror 110 and a detector 102. The light source 112 is configured to emit light. The mirror 110 is configured to receive the light from the source 112 and redirect or reflect the light to the detector 102.
System 100 also includes two mechanisms that are used for calibrating detector 102. These mechanisms include a source assembly rotation mechanism 104 and a mirror rotation mechanism 106. The source assembly rotation mechanism 104 rotates or moves light source 112 to illuminate mirror 110 and thus detector 102. That is, the source assembly rotation mechanism 104 directs the Field of View (FOV) of detector 102 at the source 112. The mirror rotation mechanism 106 rotates or moves mirror 110 into the incoming light path from source 112 so that the mirror 110 receives the light from the source 112, and redirects that light onto the detector 102.
During calibration mode of the system 100, as shown in FIG. 2, the source 112B is moved or rotated by the source assembly rotation mechanism 104, and the mirror 110 is moved or rotated by the mirror rotation mechanism 108 such that the light emitted by the source 112B is received by the mirror 110, and the received light is reflected by the mirror 110 onto the detector 102. Also, during calibration mode, the source 112A is moved or rotated such that the light emitted by the source 112A is received by the mirror 110 and reflected by the mirror 110 onto the detector 102, finally the source 112C is moved or rotated such that the light emitted by the source 112C is received by the mirror 110 and reflected by the mirror 110 onto the detector 102 to complete the calibration sequence.
These two discrete mechanisms (i.e., the source assembly rotation mechanism 104 and the mirror rotation mechanism 106) used for calibrating the detector 102 add complexity to the system 100 and introduce additional reliability concerns over a single mechanism. Embodiments of the present disclosure provide improvements over the conventional calibrating systems.